HIGH SPEED InAlAs/InGaAs DOUBLE HETEROSTRUCTURE p-i-n's

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HIGH SPEED InAlAs/InGaAs DOUBLE HETEROSTRUCTURE p-i-n’s

J.-C. Bischoff, T. Hollenbeck, R. Nottenburg, M. Tamargo, J. de Miguel, C.

Moore, H. Schumacher

To cite this version:

J.-C. Bischoff, T. Hollenbeck, R. Nottenburg, M. Tamargo, J. de Miguel, et al.. HIGH SPEED

InAlAs/InGaAs DOUBLE HETEROSTRUCTURE p-i-n’s. Journal de Physique Colloques, 1988, 49

(C4), pp.C4-329-C4-332. �10.1051/jphyscol:1988469�. �jpa-00227967�

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HIGH SPEED InAlAs/InGaAs DOUBLE HETEROSTRUCTURE p-i-n's

J.-6.

B I S C H O F F ( ~ ) , T.H. HOLLENBECK",('), R.N. N O T T E N B U R G * - ( 3 ) ,

M. C. TAMARGO*

,

J. L

.

^DE^MIGUEL*

^,

C. F. MOORE" and H. SCHUMACHER*

Institute for Micro and Optoelectronics, Swiss ~ e d e r a l Institute of Technology, CH-1015 Lausanne, Switzerland

e ell Communications Research Inc., 331, Newman Spring Rd., Red Bank, NJ 07701.

U.S.A.

Resume

-

Des photodiodes p-i-n InAlAs/InGaAs a double heterostructure ont Bte produites p a r MBE e t montees s u r des guides d'onde coplanaires. Deux methodes:

( i ) l'incorporation d e couches InAlAs non dopees e n t r e l a zone d'absorption InGaAs e t l e s couches InAlAs dopees e t ( i i ) l'utilisation d'heterojonctions graduelles o n t e t e utilisbes pour reduire l'accumulation de porteurs a u x interfaces. Bien que les deux methodes ameliorent l e s performances des diodes p-i-n a double heterojonction, les meilleurs r k s u l t a t s o n t e t e o b t e n u s avec l e s heterojonctions graduelles: une efficacite q u a n t i q u e de 238 % a 1.3 um. u n temps de montee de 2 1 ps e t une largeur a mi-hauteur de 40 ps ont e t e obtenus pour une diode a y a n t une surface de 24x24 pmz e t u n e couche d'absoption d'une epaisseur d e 0.5 um.

A b s t r a c t

-

MBE grown double h e t e r o s t r u c t u r e InAlAs/InGaAs p-i-n photodiodes h a v e been flip-chip mounted on coplanar waveguides. Two methods: ( i ) incorporation of doping setbaclt InAlAs l a y e r s between t h e InGaAs absorption region and t h e doped InAlAs l a y e r s and (ii) compositional grading h a v e been used t o reduce c a r r i e r pile-up a t t h e heterointerfaces. Although both methods improve t h e diode performances. t h e b e s t r e s u l t s were obtained with compositional grading: a quantum efficiency of

=38 % a t 1.3 urn, a r i s e time of =21 ps a n d a FWHM of =40 ps h a v e been obtained for a device with a 24x24 urn2 a r e a and a 0.5 pm thick absorption region.

One of t h e most promising approaches for d e t e c t i o n in t h e 1.0

-

1.7 urn wavelength range i s t h e use of a p-i-n photodiode coupled t o a field e f f e c t t r a n s i s t o r amplifier.

Ino.goCa0.47As based p-i-n photodiodes h a v e already shown low d a r k c u r r e n t , good quantum efficiency a n d high speed operation /1//2//3/. Most devices i n v e s t i g a t e d t o d a t e a r e of t h e homojunction t y p e with a p-type l a y e r formed by diffusion of Zn in t h e J I n C a A s layer. In

c o n t r a s t , t h e devices i n v e s t i g a t e d here a r e MBE grown I ~ o . ~ z A ~ o . ~ ~ A ~ / I ~ o . ~ ~ C ~ O . ~ ~ A ~ / I ~ O . ~ A ~ O . ~ ~ A S double h e t e r o s t r u c t u r e p-i-n's.

Immediate a d v a n t a g e s of a double h e t e r o s t r u c t u r e a r e a n excellent control of t h e absorption region t h i c k n e s s and t h e elimination, l n t h e photoresponse, of t h e diffusion t a i l due t o absorption in low field segions. The pulse response of t h e s e devices is however, typically characterized by long t a i l s d u e to carrier pile-up (trapping) a t t h e heterojunction i n t e r f a c e s /4/. We p r e s e n t r e s u l t s of a n investigation o'f two possible methods t o reduce t h e deleterious e f f e c t s of c a r r i e r trapping: ( i ) incorporation of doping setback l a y e r s a t t h e h e t e r o i n t e r f a c e s a n d ( i i ) compositional grading of t h e heterointerfaces.

(')work done while at Bell Comunications Research Inc., Red Bank, NJ 07701. U.S.A.. as a resident visitor (')present address : Princeton University. Princeton. NJ 08544. U.S.A.

(3)~resent address : AT&T Bell Laboratories. Murray Hill, NJ 07974, U.S.A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988469

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JOURNAL DE PHYSIQUE

The samples were grown by Molecular Beam Epitaxy (MBE) on InP (Fe-doped) semi-insulating s u b s t r a t e s . Three t y p e s of s t r u c t u r e s were realized. The f i r s t o n e i s a double heterojunction p-i-n s t r u c t u r e with a b r u p t interfaces which consists of an undoped Ino.sctGao.4tAs absorption l a y e r sandwiched by two Ino.szAlo.~eAs layers, one doped with Si (n-type) and t h e o t h e r with Be ( p - t y p e ) . The two o t h e r s a r e s l i g h t l y modified s t r u c t u r e s . One contains 300 A setback layers of undoped In0.s~Alo.4sAs separating t h e doped layers from t h e intrinsic In0.33Ga0.47As a n d t h e o t h e r one, compositionally grzded (In, Ga, Al) As l a y e r s gradually going from t h e Ino.~zAl0.48As composition t o t h e Ino.ssGao.47As. These graded l a y e r s a r e 300 A thick and undoped.

Mesa diodes, designed for back illumination through t h e semi-insulating InP s u b s t r a t e . were fabricated from t h e as-grown wafers and mounted, upside down, on coplanar waveguides. With t h i s technique, t h e losses due to t h e coupling of t h e p-i-n diode to t h e coaxial connector ('Weltron K connector) c a n be maintained below' 3 dB a t 26 GHz.

Dark c u r r e n t s of respectively 328 pA and 537 pA a t 5 V were measured. for devices with compositional grading and a r e a s of respectively 14x14 pm2 a n d 24x24 pmz. For t h e s e devices, whose absorption l a y e r t h i c k n e s s i s 0.5 pm, t h e c a p a c i t a n c e s a t 0 V bias a r e respectively 50 fF a n d 145 fF.

We present in fig. 1 t h e r e v e r s e I-V c h a r a c t e r i s t i c s of double h e t e r o s t r u c t u r e p-i-n diodes with a b r u p t heterointerfaces. i n t h e dark (lower c u r v e ) a n d f r o n t illuminated with white light through t h e prober microscope (upper curve). The photoresponse shows a turn-on voltage a t

=

0.4 V. Dispersion i n t h e c h a r a c t e r i s t i c s of diodes issued from different processing r u n s i n d i c a t e s t h a t t h i s turn-on voltage is s e n s i t i v e t o t h e processing and i s related t o t h e quality of t h e Schottky t y p e Ti-Au contacts. Above t h e turn-on voltage, t h e s e n s i t i v i t y of t h e photodiodes i s f a i r l y independent of t h e applied bias voltage.

In fig. 2, we h a v e reported t h e peak amplitude of t h e response of double h e t e r o s t r u c t u r e p-i-n diodes t o optical pulses generated by a c t i v e mode locking of a l a s e r diode a s a function of t h e r e v e r s e bias voltage ( c h a r a c t e r i s t i c s of pulses generated by l a s e r #1: r i s e time

=

44 ps, FWHM

=

4 5 ~ s . repetition r a t e

=

350 MHz and peak power

=

14.6 mW.). I t a p p e a r s t h a t t h e s e devices h a v e , under pulsed excitation, a behavior which differs significantly from t h a t under continuous illumination. Indeed t h e characteristics reported i n fig. 2 show t h a t our devices do not respond t o f a s t optical pulses when t h e i r reverse bias voltage i s below a threshold v a l u e which depends on t h e type of heterointerface: =1 V for devices with compositionally graded i n t e r f a c e s , ~ 2 . 8 V for devices with doping setback l a y e r s and -5 V for devices with a b r u p t interfaces. Above threshold, t h e peak c u r r e n t of t h e t h r e e t y p e s of devices increases l i n e a r l y with bias i n t h e voltage range shown in f i g . 2 . The b e s t s e n s i t i v i t y , obtained a t 6 V bias for a double h e t e r o s t r u c t u r e p-I-n with a 0.5 vm t h i c k absorption layer, is 0.4 A/W (peak amplitude = 291 mV). At 1.3 pm. t h i s corresponds to a n e x t e r n a l quantum efficiency of 38%.

At higher bias voltages, t h e pulse response peak amplitude levels off (fig. 3, characteristics of pulses g e n e r a t e d by l a s e r #2: r i s e time

=

5.3 ps, FWHM

=

7.4 ps, repetition r a t e

=

440 MHz and peak power

=

6 mW.). Only t h e devices with compositional grading were measured i n t h e higher voltage range. Indeed for devices with a b r u p t interfaces or doping s e t b a c k , t h e breakdown voltage.

=

8 V, was reached before a n y leveling of t h e response could be observed.

Devices with a b r u p t i n t e r f a c e s show a 80 ps r i s e time a n d a 1.4 n s f a l l time a t 5.5 V bias. This long fall time i s s t r o n g l y temperature dependent and c a n be reduced t o 180 ps by elevating t h e d e t e c t o r temperature by directing a h o t a i r flow to t h e d i o d d Devices which employ doping setback a n d compositional grading show threshold response a t bias levels of 3 V and 1.5 V respectively. Operating t h e devices, with doping setback, a t 5.5 V bias, we observe over a n order of magnitude improvement in fall time: 120 p s v e r s u s 1.34 n s for t h e diode with a b r u p t interfaces. The same observation can be made a t 3.3 V bias by comparing t h e devices having compositional grading with t h e ones having doping setback: 110 ps versus 1.02 ns.

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fall time a t 8 V bias for a n a c t i v e a r e a of 24x24 urn2 (fig. 4). Devices of smaller size h a v e not shown a n y speed improvement over t h e largest ones. This i s .due to t h e i r smaller top c o n t a c t to mesa a r e a r a t i o resulting i n a n increased s e r i e s resistance.

To e v a l u a t e t h e i n t r i n s i c rise time and FWHM of our devices, we h a v e made t h e following assumptions: t i ) t h e pulses h a v e a gaussian waveform, ( i i ) t h e S4 sampling head r i s e time is 25 ps and t h e corresponding FWHM is 3 5 ps and (iii) t h e light pulse rise time a n d FWHM a r e 5.3 ps a n d 7.4 p s /5/. Under t h e s e assumptions, we obtain for t h e device whose pulse response a p p e a r s in fig. 4 a n intrinsic rise time of 21 p s and a FWHM of 40 ps.

During t h i s work, we made t h e following observations. The dependence of t h e diode responsivity on bias voltage i s a p p a r e n t only under pulsed excitation. The responsivity of t h e diode i s larger when t h e a r e a l density of carriers trapped a t t h e heterointerface under dark conditions i s low, for example, when t h e band discontinuity i s small (compositional grading of t h e heterointerface) o r when t h e band potential minimum i s above (for electrons) t h e quasi-Fermi level ( s t r u c t u r e with doping setback layers under reverse bias). These observations lead us t o t h e assumption t h a t t h e time s p e n t by a c a r r i e r a t t h e heterointerface, and t h u s t h e probability t o be lost by recombination, decreases when t h e photogenerated c a r r i e r d e n s i t y in t h e well i s large i n comparison with t h e c a r r i e r d e n s i t y i n t h e well under dark conditions. However, f u r t h e r work will be necessary to determine e x a c t l y t h e mechanisms involved i n t h e t r a n s p o r t of carriers through band discontinuities in double h e t e r o s t r u c t u r e p-i-n diodes.

In conclusion, we h a v e presented two methods to reduce carrier trapping a t t h e heterointerfaces i n InAlAs/InGaAs double h e t e r o s t r u c t u r e p-i-n photodiodes: incorporation of doping setback InAlAs l a y e r s and of compositionally graded InAlGaAs layers. The b e s t r e s u l t s h a v e been obtained with devices with compositional grading and a r e very promising: e x t e r n a l quantum efficiency

=

3 8 96, r i s e time

=

21 ps and FWHM

-

^{40 ps.}

The a u t h o r s wish t o acknowledge R. F. Leheny and M. Ilegems for t h e i r s u p p o r t and M.-H. Meynadier, C. Caneau, B. W. Meagher, D. A. Humphrey. R. J. Martin and B. Zimmermann for t h e i r a s s i s t a n c e during t h e course of t h i s work. We a r e particularly g r a t e f u l t o H. Temkin for h i s advise and for providing t h e l a s e r diodes.

References

/1/ J . E. Bowers, C. A. Burrus, "Optoelectronic Components and Systems with Bandwidths i n Excess of 26 Ghz", RCA Review. Vol. 46, pp. 497-509, Dec. 1985.

/2/ J. E. Bowers. C. A. Burrus, "InGaAs PIN Photodetectors with Modulation Response t o Millimetre Wavelengths". Electronics Letters. Vol. 21. pp. 812-814, 29'h August 1985.

/3/ 'H. Temkin, R. E. Frahm, N. A. Olsson, C. A. Burrus, R. J. Mccoy, "Very High Speed Operation of Planar InGaAs/InP Photodiode Detectors", Electronics Letters, Vol. 22, No. 23, pp. 267-269. 6th Nov. 1986.

/4/ Y. Zebda, P. Bhattacharya, M. S. Tobin and T. B. Simpson, "Design and Performance of Very High-Speed I ~ o . ~ ~ G ~ o . ~ ~ A ~ / I ~ o . J z A ~ o . ~ ~ A ~ p-i-n Photodiodes Grown by Molecular Beam Epitaxy", IEEE Electron Device Letters. Vol. EDL-8. No. 12. December 1987. pp. 579-581.

/5/ J. P. v a n d e r Ziel, "Active mode locking of double heterostructure l a s e r s in a n e x t e r n a l cavity", J. Appl. Phys., Vol. 52. No. 7 , pp. 4435-4446, J u l y 1981.

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JOURNAL DE PHYSIQUE

Pig. 1 I-V c h a r a c t e r i s t i c s of a 24x24 pm2 double h e t e r o s t r u c t u r e p-i-n diode with a b r u p t i n t e r f a c e s in t h e dark (lower curve, I = 586 pA for V = 5.0 V) a n d illuminated with white . l i g h t through t h e prober microscope (upper curve).

300

25a

5: _Em -

- ij

^{1 3 -}

"

=

2

100-

50

0 -

Fig. 2 Dependence of peak amplitude on bias voltage of 24x24 pm2 diodes with a b r u p t i n t e r f a c e s , doping setback l a y e r s and compositional grading. Optical pulse characteristics: r i s e time

=

44 ps. FWHM

=

45 ps, repetition r a t e

=

350 MHz a n d peak power

=

14.6 mW.

o abrupt interface8 ' ,' ,,

"

i :

I /

- A doping setback .?, .

0 compositional grsd~ng

,:' $

, i

/ ^A/

-

::4 , o / , , , , .

?,'

"

0.0 1.0 2.0 3.0 4.0 5.0 8.0 7.0 8.0 9.0 10.0 Bias Voltage [VI

Fig. 3 Dependence of peak amplitude.

r i s e time a n d FWHM on bias voltage of a 24x24 prn2 with compositional grading.

Optical pulse characteristics: rise time

=

5.3 ps, FWHM z 7.4 ps, repetition r a t e z 440 MHz and peak power

=

6 mW.

. -"

118 116-

" 4 -

E

112-

g

,I,.

Fig. 4 Pulse response of a 24x24 urnZ diode with compositional grading a t 8 V bias. Optical pulse characteristics: rise time

=

⁴⁴^{ps, FWHM} ^{z 45}ps, repetition r a t e

=

350 MHz and peak power

=

^14.6mw.

Scale x: 50 ps/div.

y: 20 mV/div.,

-

^o^amplitude

A rke time o FWHM

/